Lithographic projection apparatus, a method for determining a position of a substrate alignment mark, a device manufacturing method and device manufactured thereby

ABSTRACT

Alignment to buried marks is carried out by using electromagnetic radiation to induce waves in the layers covering the buried layer. The acoustic or thermal waves cause reflectivity changes and displacements in the surface whose position and/or time dependence reveals the true position of the buried alignment mark. The buried alignment mark may be revealed by mapping the thickness of covering layers in its vicinity, e.g. by measuring the time dependence of the decay of a standing wave induced in the covering layers or by measuring the delay time of echoes of a travelling wave created at interfaces between different ones of the covering layers. Alternatively, a travelling wave can be created over the whole area of the mark so that echoes off the top and bottom of the buried mark carry positive and negative images of the mark; these cause reflectivity differences and displacements when they reach the surface which can be aligned to.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the alignment of asubstrate in a lithographic projection apparatus. More specifically, itrelates to alignment of the substrate after some process layers havebeen deposited above an alignment mark.

[0003] 2. Description of the Related Art

[0004] The term, “patterning means”, “patterning structure” or “mask” ashere employed should be broadly interpreted as referring to means thatcan be used to endow an incoming radiation beam with a patternedcross-section, corresponding to a pattern that is to be created in atarget portion of the substrate; the term “light valve” can also be usedin this context. Generally, the said pattern will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit or other device (see below).Examples of such patterning means include:

[0005] A mask. The concept of a mask is well known in lithography, andit includes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0006] A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-adressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference. In the case of a programmable mirrorarray, the said support structure may be embodied as a frame or table,for example, which may be fixed or movable as required.

[0007] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0008] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning means ashereabove set forth.

[0009] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning means may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—commonlyreferred to as a step-and-scan apparatus—each target portion isirradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally<1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat No. 6,046,792, incorporated herein by reference.

[0010] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

[0011] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference.

[0012] A very important criterion in semiconductor manufactures is theaccuracy with which the successive layers printed on the substrate arealigned with each other. Misalignments of the layers are referred to asoverlay errors, for all the many layers required to make an integratedcircuit the overlay errors must be kept within tight limits for theresulting device to function correctly. To correctly align the substrateto the mask and consequently minimize overlay errors, alignment marks,which generally take the form of diffraction gratings are etched in thebare silicon substrate. These alignment marks (referred to as “zeromarks”) are aligned to corresponding marks provided on the mask using avariety of techniques, including through the lens (TTL) alignmentsystems and off-axis alignment systems. An example of the latter isdescribed in EP-A-0 906 590 (P-0070). However, once a few process layershave been deposited or grown on the substrate, the zero marks etched inthe bare substrate often become obscured and are no longer visible tothe radiation used in the alignment process. Even if not completelyobscured, the growth of layers on top of the alignment marks can beuneven, leading to a shift in the apparent position of the alignmentmark. To enable alignment after the zero marks have been obscured,further alignment marks are printed during the deposition of suitablelayers of the device. The subsequent marks, referred to as non-zeromarks, are however subject to damage during subsequent process steps andwill also accumulate overlay errors from previous process layers. Whenetching a blanket aluminum layer to define the interconnects of theintegrated circuit, it is preferred to align to the original zero marksbut to do this requires that the overlying aluminum layers, and possiblyalso dielectric layers, be removed. Such clearout steps are undesirable.

[0013] A technique known as Impulsive Stimulated Thermal Scattering(ISTS) for measuring acoustic and thermal film properties, such aselastic constants and thermal diffusion rates, has been described invarious publications such as J. A. Rogers et al., Appl. Phys. Lett 71(2), 1997; A. R. Duggal et al. J. Appl. Phys. 72 (7), 1992; R. Logan etal., Mat. Res. Soc. Symp. Proc. 440, pg 347, 1997; L Dhar et al., J.Appl. Phys. 77 (9), 1995; and J. A. Rogers et al. Physica B 219 & 220,1996. In this method, two excitation pulses overlapping in time andspace are incident on a sample at slightly different angles. The twopulses interfere and heat the sample in a pattern corresponding to theinterference pattern between them. The local heating sets up vibrationsin the crystal structure of the sample which act as a diffractiongrating to a probe pulse incident on the sample shortly after theexcitation pulses. The diffraction of the excitation pulse is measuredto give an indication of the property being investigated in the sample.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide an alignmentsystem capable of alignment to alignment marks, e.g. formed directly inor on the substrate surface, even after they have been buried bysubsequent process steps.

[0015] According to the present invention there is provided alithographic projection apparatus including:

[0016] a radiation system for supplying a projection beam of radiation;

[0017] a support structure for supporting patterning means, thepatterning means serving to pattern the projection beam according to adesired pattern;

[0018] a substrate table for holding a substrate;

[0019] a projection system for projecting the patterned beam onto atarget portion of the substrate; and

[0020] an alignment system for aligning the substrate to the patterningmeans, characterized by:

[0021] said alignment system comprising an excitation source fordirecting electromagnetic radiation to a surface of said substrate so asto induce a wave therein in a region of an at least partially buriedsubstrate alignment mark; and a measurement system for directing ameasurement beam to be reflected by said surface and for detectingsurface effects caused by said wave thereby to perform an alignment tosaid substrate alignment mark.

[0022] The present invention uses acoustic or thermal waves induced inthe process layers covering, or partially covering, a substratealignment mark to reveal its true position. The substrate alignment markmay be one provided in or on the substrate itself or a deposited processlayer. It thereby allows accurate alignment for critical process stepsin a manufacturing procedure, without accumulating overlay errors fromearlier steps and without the need for clearout steps on layer coveringthe mark. The waves cause surface displacement and reflectiondifferences in the surface whose position and/or time dependence revealsthe true position of the buried substrate alignment mark. The buriedsubstrate alignment mark may be revealed by mapping the thickness ofcovering layers in its vicinity, e.g. by measuring the time dependenceof the decay of a standing wave induced in the covering layers or bymeasuring the delay time of echoes of a travelling wave created atinterfaces between different ones of the covering layers. Alternatively,a travelling wavefront can be created over the whole area of the mark sothat echoes off the top and bottom of the buried mark carry positive andnegative images of the mark; these cause surface displacement when theyreach the surface which can be aligned to.

[0023] According to a further aspect of the present invention there isprovided a method for determining a position of a substrate alignmentmark, including:

[0024] inducing a wave in surface layers of a substrate at leastpartially covering the substrate alignment mark;

[0025] measuring surface effects of the surface of said substrate wheresaid wave has been induced; and

[0026] determining the position of said substrate alignment mark usingthe results of said step of measuring said surface effects.

[0027] The position of the buried substrate alignment mark may bedetermined with respect to the substrate or with respect to a table onwhich the substrate is positioned. This determined position may be usedin a lithographic projection apparatus or in a monitoring apparatus formonitoring the quality of exposed substrates.

[0028] The present invention also provides a method of manufacturing adevice including the method described above and further imagingirradiated portions of the mask onto target portions of the substrate.

[0029] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “exposurearea”, respectively.

[0030] In the present document, the terms “radiation ” and “beam” areused to encompass all types of electromagnetic radiation or particleflux, including, but not limited to, ultraviolet radiation (e.g. at awavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extremeultraviolet radiation (EUV), X-rays, electrons and ions. Also herein,the invention is described using a reference system of orthogonal X, Yand Z directions and rotation about an axis parallel to the I directionis denoted Ri. Further, unless the context otherwise requires, the term“vertical” (Z) used herein is intended to refer to the direction normalto the substrate or mask surface, rather than implying any particularorientation of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

[0032]FIG. 1 depicts a lithographic projection apparatus according to afirst embodiment of the invention;

[0033]FIG. 2 depicts a zero mark on a wafer covered by an aluminum layerillustrating PVD induced alignment shift;

[0034]FIG. 3 is a view used in explaining the cause of PVD inducedalignment shift;

[0035]FIG. 4 depicts laser-induced surface gratings used in the firstembodiment of the invention;

[0036]FIG. 5 depicts a zero mark on a wafer covered by an aluminum layerillustrating PVD induced alignment shift and the corresponding layerthickness;

[0037]FIGS. 6A to 6D illustrate a thickness measurement technique usedin a second embodiment of the invention; and

[0038]FIGS. 7A to 7E illustrate the procedure for revealing a buriedmark of a third embodiment of the invention.

[0039] In the drawings, like references indicate like parts.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Embodiment 1

[0041]FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes:

[0042] a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation). In this particular case, theradiation system also comprises a radiation source LA;

[0043] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g. a reticle), and connected to firstpositioning means for accurately positioning the mask with respect toitem PL;

[0044] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g. a resist-coated siliconwafer), and connected to second positioning means for accuratelypositioning the substrate with respect to item PL;

[0045] a projection system (“lens”) PL (e.g. a refractive orcatadioptric system, a mirror group or an array of field deflectors) forimaging an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies) of the substrate W.

[0046] As here depicted, the apparatus is of a transmissive type (i.e.has a transmissive mask). However, in general, it may also be of areflective type, for example (with a reflective mask). Alternatively,the apparatus may employ another kind of patterning means, such as aprogrammable mirror array of a type as referred to above.

[0047] The source LA (e.g. a HG lamp, excimer laser, an undulatorprovided around the path of an electron beam in a storage ring orsynchrotron, a laser plasma produced source, a discharge source or anelectron or ion beam source) produces a beam of radiation. This beam isfed into an illumination system (illuminator) IL, either directly orafter having traversed conditioning means, such as a beam expander Ex,for example. The illuminator IL may comprise adjusting means AM forsetting the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in thebeam. In addition, it will generally comprise various other components,such as an integrator IN and a condenser CO. In this way, the beam PBimpinging on the mask MA has a desired uniformity and intensitydistribution in its cross-section.

[0048] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.with the aid of suitable directing mirrors); this latter scenario isoften the case when the source LA is an excimer laser. The currentinvention and Claims encompass both of these scenarios.

[0049] The beam PB subsequently intercepts the mask MA, which is held ona mask table MT. Having traversed the mask MA, the beam PB passesthrough the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed.

[0050] The depicted apparatus can be used in two different modes:

[0051] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at once (i.e. a single“flash”) onto a target portion C. The substrate table WT is then shiftedin the x and/or y directions so that a different target portion C can beirradiated by the beam PB;

[0052] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M={fraction (1/4)} or {fraction (1/5)}). Inthis manner, a relatively large target portion C can be exposed, withouthaving to compromise on resolution.

[0053]FIG. 2 shows a zero mark M0 etched in the substrate of wafer W andcovered by an aluminum layer A1. If, as illustrated, the aluminum layerAL has been deposited at an angle on the zero mark M0, the center of thealuminum-covered mark is shifted relative to the center of theunderlying zero mark M0 by an amount d. An alignment sensor effectivelydetects the position of the center of the mark and so will give aposition shifted from the true position by an amount d. Where the markis a grating, an alignment sensor effectively measures the averageposition of all the lines in the grating. However, since all the gratinglines are close together, they will all have a similarly shiftedaluminum deposition and so the average position will suffer from thesame shift.

[0054] A probable cause of asymmetric aluminum deposition in PhysicalVapor Deposition (PVD) is shown in FIG. 3 in relation to a mark M0 nearthe edge of the wafer. In PVD, the aluminum layer grows by accretion ofaluminum particles. As each particle is deposited, the layer will growin the direction of incidence of that particle. Since the layer is builtup from many particles, the net direction of growth of the layer will berelated to the average direction of incidence of the particles making upthe layer. As can be seen from FIG. 3, the average angle of incidence ofparticles on a mark M0 near the edge of the wafer will be an angle Al,which is somewhat inclined to the vertical, whereas the average angle ofincidence A2 of particles on a mark M0′ near the center of the waferwill be vertical or nearly so. Thus aluminum will grow on mark M0 at anangle toward the center of the wafer, resulting in an effective shift ofthe mark.

[0055] In the first embodiment of the invention, the thickness of thealuminum layer across the area of the mark is measured using animpulsive stimulated thermal scattering (ISTS) technique. This isillustrated in FIG. 4. Two excitation pulses EP are emitted byexcitation source 11 and directed so as to be co-incident simultaneouslyon the wafer surface at a small angle a to the normal. The twoexcitation pulses EP are of sub-nanosecond duration, e.g. 400 ps, andexcitation source 11 may be a passively Q-switched, single mode, Nd-YAGmicrochip laser pumped by a 1.2 W diode laser. The excitation pulsewavelength may be 1064 nm, for example. Suitable laser sources aredescribed in J. J. Zayhowski, Laser Focus World, April 1996 pp 73-78,which document is incorporated herein by reference. In the film(aluminum layer), thermal expansion induced by the local heating of thefilm where the two excitation pulses constructively interfere inducesacoustic and thermal responses, leading to the formation of a thermalgrating. The acoustic waves are counter-propagating and damped so thethermal waves forming the grating are quasi-steady state materialresponses and persist until the thermal diffusion washes out.

[0056] While the thermal grating persists, a probe pulse PP is emittedby probe source 12, at a relatively large angle to the normal, so as tobe diffracted by the thermal grating. The amount of diffraction of theprobe pulse PP is detected by detector 13, which allows the state of thethermal diffraction grating to be monitored in real time. The probesource 12 may be an 860 nm diode laser operating in a quasi-continuouswave mode. The detector, and data recording/processing electronics havea nanosecond time resolution.

[0057] The excitation region is typically 25 μm×500 μm while the probespot can be circular with 20 μm diameter, allowing several measurementsto be taken from one excitation.

[0058] The fringe spacing of the induced grating depends only on thewavelength of the excitation pulses and the crossing angle a. The filmacts as an acoustic waveguide, supporting waveguide modes whosedisplacements include both shear and compression. Each mode has acharacteristic dispersion relation giving the acoustic velocity as afunction of wave vector. Each mode has the same dependence on the wavevector and the film thickness; the dispersion relation is determined bythe elastic modulus and density of the film and the underlyingsubstrate. Since the properties of the, e.g. AlCu, film are known, inthe invention, the time-dependent diffraction of the probe beam can beused to determine the acoustic frequency and hence film thickness.

[0059] A plurality of spaced apart film thickness measurements are takenalong a line bisecting the alignment mark. As shown in FIG. 5, thethickness profile of the Al layer will show one thinner region, t1, andone thicker region, t2, either side of the zero mark M0. The thinner andthicker regions t1, t2 correspond to the Al deposits on zero mark M0 andtheir width will indicate the apparent alignment shift observed whenaligning to the obscured mark. The determined widths can be used tocorrect an alignment carried out to the surface appearance of the mark.

[0060] Embodiment 2

[0061] In a second embodiment of the invention a different method isused to measure the film thickness, but otherwise the same principlesapply. This method is illustrated in FIGS. 6A to 6D.

[0062]FIG. 6A shows pump source 21, which may be a TiS laser for exampleand emits ultra short excitation pulses EP, for example pulses of 150 fsduration at a frequency of 80 MHz, which are directed onto the wafer Wwhere they instantaneously heat the surface of the uppermost layer L1 onthe wafer at spot HS. The heating of the surface creates an acousticwave S which propagates downwards into the layers L1, etc, deposited onthe wafer substrate W, as shown in FIG. 6B. Meanwhile, detection beamsource 22 directs detection beam DB onto the wafer surface where it isreflected to detector 23, whose output is a measure of the surfacedisplacement of layer L1. The detection beam DB may be a delayed portionof the excitation pulses EP or may be generated by a separate source.

[0063] When the acoustic wave S reaches the first interface in thestack, between layers L1 and L2, a portion of the energy will bereflected back towards the surface, shown as echo E1 in FIG. 6C, whilethe attenuated acoustic wave S continues downwards. The proportion ofenergy reflected will depend on the acoustic impedances of the twolayers. When echo E1 reaches the upper surface of the top layer L1 asshown in FIG. 6D, it will cause a displacement and a change inreflectivity of that surface. The change in reflectivity or thedisplacement is detected by detector 23. The sign and magnitude of thedisplacement will depend on the two materials meeting at the interfaceand factors such as the roughness of the interface (the local crystalstructure). Of course, as the acoustic wave S propagates further downthe layers deposited on the wafer, other echoes will be generated. FIG.6D also shows echo E2 generated at the interface between layers L2 andL3.

[0064] The timing of the displacements and the changes in reflectivityare dependent on the speed of sound in the layers and the layerthickness; since the former are known the later can be calculated quitesimply.

[0065] One could, for example, use a 15 femtosecond pulse EP of 5 nJ perpulse focussed down to a spot HS having a 20 μm diameter. This pulse ispartly absorbed leading to an instantaneous, local heating ofapproximately 50 degrees on the surface of the toplayer L1. The pulsefunctions as a microscopic hammerblow giving an acoustic pulse Spropagating further into the medium. The acoustic pulse is a strainpulse which means that over the pulsewidth the medium is slightlydeformed with a relative length variation (or strain) Δ1/1 in alldirections. The maximum strain is equal to the thermal expansion at thestart (Δ1/1=β* ΔT where β=2.3*10⁻⁵K⁻¹ is the thermal expansioncoefficient of aluminum). The strain pulse is partially reflected fromevery interface including the interface between the deposited layer L3and the wafer W where the buried mark is etched. The reflected pulsereturns to the surface of the top layer L1 where it will give a surfacedisplacement and a variation in reflection. This can be measured with adetection beam source 22 and detector 23. The coefficient of surfacereflection per unit strain for aluminum is approximately 2*10⁻³. Themaximum change in surface reflection is then 2*10⁻³*β*ΔT≈10⁻⁶. Thesurface displacement is equal to the maximum strain times the pulselength: β*ΔT*ξ≈10⁻¹¹ wherein ξ is the absorption length (15.1 nm). Itmust be noted that this calculation model assumes no energy losses,however it is expected that energy losses will occur and therefore themeasured changes in reflectivity and surface displacement will besmaller.

[0066] In variations of the second embodiment, the displacement data canbe processed to compensate for a relatively large spot size, and thespot size can be reduced using a second grating to blade part of themark structure.

[0067] Embodiment 3

[0068] In a third embodiment of the invention, the buried mark isacoustically revealed on the surface of the covering layers and can thenbe directly aligned to. The procedure for this is shown in FIG. 7A to7E.

[0069] First, the outer surface OS of the deposited layer or layerscovering the mark M is excited using a short pulse laser, for example ofthe type described above, over the whole area of the buried mark M. Thisgenerates an acoustic wavefront WF which propagates downwards throughthe covering layers, as shown in FIG. 7A. When the wavefront WF meetsthe level of the top of the buried mark M, as shown in FIG. 7B,reflections will be generated only in the areas where the mark israised. Thus the first reflection R1 which returns towards the outersurface OS will carry an image of the buried mark. The remainder of thewavefront WF continues to propagate downwards in the etched area of themark M. This is the situation shown in FIG. 7C.

[0070] When the first reflection R1 reaches the outer surface OS, asshown in FIG. 7D, the surface will be displaced and the reflectivitywill be changed in a pattern corresponding to the buried mark M. Thedisplacements and the difference in reflectivity between the displacedand not-displaced areas of the surface form a diffraction grating whichdiffracts the alignment beam in the same way as the mark M itself. Analignment can then be carried out to the acoustic representation of theburied mark M.

[0071] A second alignment is also possible using the second reflectionR2, that is, a negative image of the mark may be used instead of, or inaddition to, the positive image as described above. This is reflected bythe etched away portions of the mark M and reaches the outer surface OSa short time after the first reflection RI. The time delay will dependon the depth of mark M and the speed of sound in the covering layers.FIG. 7E shows how the second reflection displaces the outer surface OSin a grating pattern that is the negative of the mark M but can bealigned to in a similar manner.

[0072] Of course, the excitation and alignment process can be repeatedas often as required to complete an alignment process to the desiredaccuracy.

[0073] In a variation of the third embodiment, the femtosecond laserused to excite the acoustic travelling wave in the layer(s) covering theburied mark is replaced by a less-expensive amplitude-modulated (semi-)continuous laser. The amplitude modulation of the continuous laser isarranged so as to periodically excite the surface layer in-phase withthe returning acoustic waves from the spaces of the buried mark and 180°out of phase with the acoustic wave returning from the lines of theburied mark and from the bulk material. The acoustic projection of themark on the surface, defined by reflectivity changes, then has a goodcontrast and can be aligned to easily. In the case of a mark buried at adepth of 120 nm in material with a speed of sound of 2.4 km/s, themodulation frequency is of the order of (2.4 ×10³/240×10⁻⁹) Hz=10 GHz.This can easily be achieved with electro-optical modulators that can betuned as appropriate for different depths of the buried mark anddifferent covering materials.

[0074] Embodiment 4

[0075] In a fourth embodiment, the buried mark is revealed by probingthe surface of the substrate with a thermal wave. A thermal wave isdefined as a harmonically varying temperature distribution in a medium,generated by illumination of the surface with a harmonically modulatedintensity from, for example, a harmonically modulated CW laser. One ofthe advantages of thermal waves is that an intensity modulated CW laseris much cheaper and easier to implement than a high power pico- orfemtosecond laser as is used as a source for acoustic waves.

[0076] A thermal wave is not a wave in the classical sense, it is asolution of a diffusion equation with a harmonically varying source termand not of the wave equation. It has no wavefront and no reflection andrefraction at an interface because it has no directionality. The thermaldistribution in a multilayer configuration will be determined by thethermal diffusion length in the separate layers. With a thermal wave thethermal properties of the medium will be measured while with an acousticwave the mechanical (elastic) properties of the sample will be measured.The harmonically varying source can be used as an alternative to theexcitation source 11 in FIG. 4, the pump source 21 of FIG. 6A, or may beused to excite the outer surface OS in FIG. 7. Just like acoustic waves,thermal waves lead to reflectance variation and displacements of thesurface that can be measured.

[0077] Aluminum and silicon have a much higher thermal conductivity andtherefore a much higher thermal diffusion length than silicon oxide thatis an effective thermal insulator. This is advantageous, since we maylike to measure a mark present in a substrate of silicon that is buriedunder a first layer of silicon oxide and a top layer of aluminum. Thesilicon oxide will function as an effective thermal insulator so thatthe temperature increase for the aluminum top layer will be relativelyhigh. For example, it is possible to measure a temperature increase forthe aluminum layer of 100 degrees for an absorbed power of 10 mW over a10 μm spot size with a 100 kHz intensity modulation frequency. Thediffusive character of the thermal waves makes that the thermaldiffusion length is inverse proportional to the modulation frequency.The heating of the aluminum surface is 30 degrees at 1 MHz and 2 degreesat 10 MHz. The increase in temperature will lead to a large thermalexpansion and a strain (a relative length variation Δ1/1) of β* ΔT whereβ2.3*10⁻⁵K⁻¹. The wafer will be harmonically expanding and shrinkingonly limited by the boundary condition that the strain at the backsideof the wafer where the wafer is fixed to the substrate table must bezero. The aluminum surface displacement is 0,5* β*ΔT*d_(w)≈1μm whered_(w) is the wafer thickness The difference in temperature of the linesand the spaces of the buried mark may be 10 degrees, leading to adifference in surface displacement of 100 nm, a height profile that canbe easily measured by a sensor.

[0078] The surface displacement using a thermal wave is much higher andtherefore easier to measure than for an acoustic wave. So by measuring aheight profile that is created by probing a buried mark with a thermalwave we can measure the position of said mark. It is also possible tomeasure the reflectance variation that is caused by the 10 degreestemperature difference. Having a reflectance temperature coefficient of3.7*10⁻⁵K⁻¹ the reflectance variation will be 3.7*10⁴. Again it must benoted that this calculation model assumes no energy losses, however itis expected that energy loses will occur and therefore the measuredchanges in reflectivity and the surface displacement will be smaller.

[0079] While we have described above specific embodiments of theinvention it will be appreciated that the invention may be practicedotherwise than as described. The description is not intended to limitthe invention. In particular, it will be appreciated that while theinvention has been described in terms of alignment to buried zero marks,it can of course be used in alignment to any buried mark or feature.

1. A lithographic projection apparatus comprising: a radiation systemconstructed and arranged to supply a projection beam of radiation; asupport structure constructed and arranged to support beam patterningstructure, the beam patterning structure serving to pattern theprojection beam according to a desired pattern; a substrate tableconstructed and arranged to hold a substrate; a projection systemconstructed and arranged to project the patterned beam onto a targetportion of the substrate; and an alignment system to align the substrateto the beam patterning structure, said alignment system comprising: anexcitation source for directing electromagnetic radiation to a surfaceof said substrate so as to induce a wave therein in a region of an atleast partially buried substrate alignment mark; and a measurementsystem to direct a measurement beam to be reflected by said surface andto detect surface effects caused by said wave thereby to perform analignment to said substrate alignment mark.
 2. Apparatus according toclaim 1, wherein: said excitation system is arranged to induce waves insaid substrate at a plurality of points; and said measurement system isarranged to detect surface effects at said plurality of points togenerate thickness data relating to thickness of at least one layercovering said substrate alignment mark and to correct an alignmentprocess carried out using a surface pattern induced by said substratealignment mark.
 3. Apparatus according to claim 1, wherein saidexcitation source is a laser constructed and arranged to emit pulsesshorter than 1 nanosecond to induce an acoustic wave in at least onecovering layer obscuring said substrate alignment mark.
 4. Apparatusaccording to claim 3, wherein said excitation source is arranged toirradiate a measurement area with two temporally coincident overlappingexcitation pulses having mutually different angles of incidence, therebyto induce a standing acoustic wave pattern in the surface of saidsubstrate; and said measurement beam is to be diffracted by saidstanding wave pattern and the measurement system detects thetime-dependent diffraction of said measurement beam.
 5. Apparatusaccording to claim 3, wherein said excitation source is arranged toirradiate a measurement area with an excitation pulse or pulse train soas to generate an acoustic travelling wave propagating into saidsubstrate; and said measurement system further comprises a measurementbeam source to direct a measurement beam to be reflected by the surfaceof said substrate and a detector for detecting time-dependent surfaceeffects of said surface of said substrate caused by returning echoes ofsaid travelling wave.
 6. Apparatus according to claim 3 wherein saidexcitation source is arranged to irradiate said region so as to inducean acoustic travelling wave in at least one covering layer obscuringsaid substrate alignment mark so as to be selectively reflected by saidsubstrate alignment mark; and said alignment system further comprises analignment beam source to direct an alignment beam to be diffracted byimages of said substrate alignment mark formed in the surface of saidcovering layer by returning echoes of said substrate alignment mark. 7.Apparatus according to claim 1, wherein said excitation source is amodulated continuous wave source which is constructed and arranged toemit a harmonically varying beam of radiation so as to induce a thermalwave in the at least one covering layer obscuring said substratealignment mark.
 8. Apparatus according to claim 7, wherein said wavesource is a continuous wave laser.
 9. Apparatus according to claim 7,wherein said wave source is modulated with a frequency lower than 10MHz.
 10. Apparatus according to claim 1, wherein said measurement systemis constructed and arranged to measure changes in reflection of thesurface of said wafer caused by waves induced by the excitation source.11. Apparatus according to claim 1, wherein said measurement system isconstructed and arranged to measure displacements of the surface of saidwafer caused by waves induced by the excitation source.
 12. Apparatusaccording to claim 1, wherein the support structure comprises a masktable to hold a mask.
 13. Apparatus according to claim 1 wherein theradiation system comprises a radiation source.
 14. A method fordetermining a position of a substrate alignment mark, comprising:inducing a wave in at least one surface layer of a substrate at leastpartially covering the substrate alignment mark; measuring surfaceeffects of the surface of said substrate where said wave has beeninduced; and determining the position of said substrate alignment markusing the results of said measuring said surface effects.
 15. A methodaccording to claim 14 wherein said inducing a wave and said measuringthe surface effects are repeated at a plurality of positions in theregion of said substrate alignment mark so as to generate a map of thethickness of at least one layer covering said substrate alignment markand said map is used in said step of determining the position of saidsubstrate alignment mark.
 16. A device manufacturing method comprising:providing a substrate provided with an alignment mark that is at leastpartially covered by a layer of radiation sensitive material; projectinga patterned beam of radiation onto a target portion of the layer ofradiation-sensitive material; and determining a position of a substratealignment mark, comprising: inducing a wave in at least one surfacelayer of a substrate at least partially covering the substrate alignmentmark; measuring surface effects of the surface of said substrate wheresaid wave has been induced; and determining the position of saidsubstrate alignment mark using the results of said measuring saidsurface effects.
 17. A device manufactured according to the method ofclaim 16.